Can I load a bunch of images in a folder & do a quick conversion from a high res image to a lo res image(s) all in one, at one batch?

Would it be done best/quicker in Light room and/or PS?

If you have catalogued images in Lr, then you can export any you select in whatever resolution you want. Photoshop is basically a single image editor, it isn't set up for this. You can cartainly do it from PS, but it is more complex.